1. Field of the Invention:
This invention relates to an anti-skid control system for a motor vehicle which is equipped with a X-type two-channel brake apparatus wherein one of the two channels includes brake apparatus associated with the front righthand and rear lefthand wheels which are disposed in diagonal relationship with each other, and the other channel includes brake apparatus associated with the front lefthand and rear righthand wheels which are disposed in diagonal relationship with each other.
2. Description of the Prior Art:
Generally, with an anti-skid control system for motor vehicles, anti-skid control is effected by means of microcomputers such that hold valves and decay valves comprising electromagnetic valves are opened and closed on the basis of electrical signals representing wheels speeds sensed by wheel speed sensors, thereby increasing, holding or reducing the brake pressure, for the purpose of securing improved steering performance and running stability of the motor vehicle, while at the same time shortening the braking distance, as disclosed in U.S. Pat. No. 4,752,104 and in FIG. 1 of the accompanying drawings of this application.
In the case where such an anti-skid control system is applied to a motor vehicle equipped with X-type two-channel brake apparatus which is arranged such that the front righthand and rear lefthand wheels are braked through a first common brake fluid pressure piping channel while the front lefthand and rear righthand wheels are braked through a second common brake fluid pressure piping channel, the lower one of the velocities of the two wheels associated with respective one of the two hydraulic pressure piping channels is selected (referred to as "select low" hereinafter) and used as channel speed which represents the speeds of the wheels associated with the respective channel, and anti-skid control such as mentioned above is effected on the basis of such channel speeds (referred to as "two-channel control" hereinafter).
With a front wheel drive type motor vehicle including a conventional cross piping type (X-type) two-channel brake system, due to the fact that the weight proportion of the front wheels is higher than that of the rear wheels, a great braking force is produced when the front wheel speed is selected as the channel speed for each cycle of anti-skid control such as mentioned above so that the braking distance can be reduced, whereas when the rear wheel speed is selected as the channel speed, only a small braking force is generated so that the braking distance tends to be increased.
Especially, for a road surface which represents different friction coefficients (.mu.) between the lefthand and righthand side portions thereof (referred to as "split .mu. road surface" hereinafter), if control is effected by always using, as the channel speed, the lower one of the speeds of one of the front wheels and the rear wheel which is disposed in diagonal relationship therewith as is the case with the prior art (referred to as "diagonal select low" hereinafter), then the front wheel running on the higher .mu. road surface is controlled with a hydraulic pressure rate corresponding to the wheel speed of the rear wheel running on the lower .mu. road surface so that the braking force applied to the front wheel turns out to be short and as a result the braking distance tends to be increased. On the other hand, two-channel control of the type that the higher one of the rear lefthand and righthand wheel speeds and the lower one of the respective front wheel speeds are always selected as the channel speeds, is disadvantageous in that the rear wheel running on the lower .mu. portion of the road surface is liable to be locked.